Temperature Belts

Table of Contents
1. Temperature belts of the world
2. Factors affecting temperature patterns on the globe
3. Mean annual temperature distribution and isotherms
4. Inter-Tropical Convergence Zone (ITCZ)
5. Seasonal temperature distribution
6. Vertical distribution of temperature
7. Temperature anomaly
8. Mean thermal equator
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Sun is the primary source of energy for the Earth's atmosphere. The atmosphere itself receives only a small fraction of the Sun's short-wave radiation directly; most of the heat available to the atmosphere comes from long-wave radiation emitted by the warmed Earth. Heating and cooling of the atmosphere occur by direct solar radiation and by energy transfer from the Earth through conduction, convection and terrestrial radiation.

Temperature belts of the world

The global surface may be broadly divided into three primary heat zones based on the mean annual distribution of temperature and the movement of the Sun's vertical rays. These zones are:

• Torrid Zone (Tropical Zone)
• Temperate Zone
• Frigid Zone

These zones are demarcated primarily by their latitudinal distance from the Equator and by the incidence of the Sun's vertical rays.

Torrid Zone (Tropical Zone)

The Torrid Zone extends from the Tropic of Cancer at 23.5°N, across the Equator (0°), to the Tropic of Capricorn at 23.5°S. This is the hottest zone of the Earth because the Sun's rays fall nearly vertically here at least once a year. Regions within this belt receive high annual insolation and typically show high mean temperatures with relatively small annual temperature ranges.

Temperate Zone

There are two Temperate Zones, one in each hemisphere, lying between 23.5° and 66.5° latitude. These zones experience moderate climates with distinct seasons. Solar altitude varies substantially through the year, producing marked seasonal contrasts in temperature.

Frigid Zone

The Frigid Zones lie poleward of the Arctic Circle at about 66.5°N and the Antarctic Circle at about 66.5°S. These regions receive very low annual insolation and are characterised by very low temperatures and long periods of snow and ice cover. In polar areas there may be months without direct sunlight (polar night) and months with continuous daylight (midnight sun).

Importance of the heat zones

Dividing the Earth into heat zones helps explain global climate patterns, seasonal behaviour, and the broad distribution of biomes and human activities. It provides a first-order framework for understanding how solar geometry controls temperature distributions worldwide.

Factors affecting temperature patterns on the globe

Temperature distribution at any place on Earth is controlled by several interacting factors. The most important are:

• Latitude
• Altitude
• Transparency of the atmosphere (aerosols, dust, water vapour, clouds)
• Land-sea distribution (continentality and maritime influence)
• Ocean currents
• Distance of the Earth from the Sun (orbital position)
• Sunspots and solar activity
• Type of land surface (vegetation, urban surfaces)
• Aspect (slope orientation)

Latitude

Latitude is the principal control on the amount of solar energy received. Places near the Equator receive higher mean solar radiation because the Sun's rays strike more nearly vertically; polar regions receive slanting rays that are spread over a larger area and pass through more atmosphere, so they receive less energy. Consequently, temperature generally decreases with increasing distance from the Equator.

• Higher temperatures near or at the Equator.
• Lower temperatures towards the poles (North and South).

Transparency of the atmosphere

The transparency of the atmosphere determines how much solar radiation reaches the surface. Transparency is affected by aerosols (smoke, soot), dust, water vapour and cloudiness.

• If the wavelength of incoming radiation is larger than the radius of atmospheric particles (e.g., gases), scattering predominates.
• If the wavelength is smaller than the obstructing particle (e.g., dust), reflection or absorption can be significant.
• Absorption occurs when particles such as water vapour, ozone or carbon dioxide and clouds intercept radiation, reducing insolation at the surface.
• Much of the sunlight reaching the surface is modified by scattering and partial absorption in the atmosphere.

Land-sea differential

Land and water respond differently to incoming solar energy because of differences in specific heat, transparency and mixing.

• Albedo (reflectivity) of land surfaces varies widely; snow-covered land can reflect 70-90% of insolation while many soils and vegetated surfaces reflect much less. Open water generally has low albedo.
• Sunlight penetrates water to depths of tens of metres, so heat is distributed vertically by mixing; on land, sunlight heats only the uppermost centimetres to metres and the surface temperature changes more rapidly.
• Consequently, land heats and cools more rapidly than oceans; continental interiors experience larger diurnal and annual temperature ranges than coastal regions.

Earth's distance from the Sun

The Earth's orbit is slightly elliptical; therefore solar distance varies through the year.

• Aphelion: the Earth is farthest from the Sun (about 152 million km) around 4 July.
• Perihelion: the Earth is nearest the Sun (about 147 million km) around 3 January.
• Insolation at perihelion is therefore slightly greater than at aphelion, but this small variation is overridden by distribution of land and sea and by atmospheric circulation; it does not produce the primary pattern of seasons.

Sunspots and solar activity

Sunspots are temporary dark (relatively cooler) markings on the Sun's surface associated with magnetic disturbances. The number of sunspots varies over an approximately 11-year cycle. Solar activity and associated phenomena (flares, faculae) alter the solar irradiance slightly; increased sunspot activity is often associated with a small rise in total solar output, which can influence climate on long time-scales.

Altitude

Altitude is the height above mean sea level. Temperature decreases with height in the troposphere.

• Higher altitudes (mountain regions) have lower temperatures than surrounding lowlands.
• Lower altitudes (plains, coastal lowlands) are generally warmer.
• The average environmental lapse rate in the troposphere is about 6.5°C per kilometre, although the actual lapse rate varies with humidity and local conditions.

Distance from the sea - continentality and maritime influence

The proximity of a place to large water bodies moderates temperature because water has high specific heat and mixes vertically.

• Maritime influence: Coastal regions have milder summers and warmer winters compared to inland locations at the same latitude because the sea warms and cools more slowly than land.
• Continental influence: Interior regions of large continents, far from the sea, experience more extreme temperatures - hotter summers and colder winters - and larger diurnal and annual ranges.

Ocean currents

Ocean currents are large-scale movements of seawater driven by wind patterns, the Coriolis force, temperature and salinity differences, and basin geometry. They transfer heat horizontally across the oceans and influence adjacent coastal climates.

• Warm currents (moving poleward) raise coastal temperatures and can produce milder winters on adjoining land.
• Cold currents (moving equatorward) lower coastal temperatures and can make coastal climates cooler and drier.
• Examples include the Gulf Stream and North Atlantic Drift (warming northern Europe), and the cold Peru (Humboldt) Current off South America (cooling western South America).

Types of land surface

Surface cover affects how much solar energy reaches and is retained by the ground.

• Dense forest: Vegetation shades the ground and evapotranspiration cools the air; surface temperatures remain relatively low.
• Urban areas: Built surfaces (concrete, asphalt) absorb and retain heat, creating urban heat islands where air temperatures are higher than surrounding rural areas.

Aspect (slope orientation)

Aspect is the compass direction that a slope faces. It affects the amount of solar radiation received.

• Aspect is more influential in mid- and high-latitude regions where solar elevation varies strongly through the year.
• In the Northern Hemisphere, south-facing slopes receive more solar radiation and are generally warmer and drier than north-facing slopes; the reverse is true in the Southern Hemisphere.
• In tropical regions, aspect has less effect near midday when the Sun is high in the sky, but it can still influence microclimates on steep slopes.

Mean annual temperature distribution and isotherms

Isotherm - an imaginary line joining places having equal temperature - is used to show horizontal (latitudinal) distribution of temperature on maps. When preparing isotherm maps, temperatures are usually reduced to sea level to remove the direct effect of altitude.

General characteristics of isotherms:

• Isotherms generally follow parallels: There is a close correspondence between isotherms and lines of latitude because solar geometry is the principal control on average insolation.
• Bends at ocean-continent boundaries: Differential heating of land and sea causes isotherms to bend where landmasses meet oceans.
• Spacing of isotherms: Narrow spacing indicates a rapid change of temperature over short distance (high thermal gradient); wide spacing indicates a gradual change (low gradient).

General patterns of temperature distribution:

• Highest absolute temperatures occur in the tropics and subtropics where insolation is high; lowest temperatures occur in polar and subpolar regions.
• Diurnal and annual temperature ranges are greatest in continental interiors due to the absence of maritime moderation.
• Diurnal and annual ranges are smallest over the oceans because of the high specific heat of water and vertical mixing.
• Low temperature gradients are observed over the tropics; higher gradients occur in middle and high latitudes where the Sun's apparent path varies strongly with seasons.
• Temperature gradients are often low along eastern margins of continents where warm ocean currents predominate, and high along western margins where cold currents are present.
• Isotherms are more irregular in the Northern Hemisphere because land area is larger there; the thermal equator (zone of highest mean temperatures) generally lies slightly north of the geographic Equator in many longitudes.
• Mountains modulate horizontal temperature distribution by blocking or channeling oceanic influences and by producing cold highland climates inland (e.g., the Rockies and Andes).

Inter-Tropical Convergence Zone (ITCZ)

The Inter-Tropical Convergence Zone (ITCZ) is a broad belt of low pressure near the equator where the northeast and southeast trade winds converge. The ITCZ closely follows the position of the Sun's vertical rays and therefore migrates north and south seasonally. It is associated with persistent convective cloudiness and high rainfall in the equatorial belt.

Seasonal temperature distribution

To understand seasonal variation in global temperature it is useful to compare isotherm maps for January and July, the months of approximate temperature extremes in the Northern Hemisphere.

• Isotherms are usually more irregular in January than in July, particularly in the Northern Hemisphere, because of the greater extent of land and stronger continental cooling in winter.
• The latitudinal control on temperature is visible in both months but is modified substantially by land-sea contrasts, ocean currents and atmospheric circulation.

Seasonal distribution - January

• January corresponds to winter in the Northern Hemisphere and summer in the Southern Hemisphere.
• The western margins of continents in the Southern Hemisphere are often warmer relative to their eastern counterparts because warm currents and prevailing winds transport heat poleward along western margins.
• Isotherms in the Southern Hemisphere tend to be more parallel to the parallels of latitude because oceanic influence is stronger there.

Northern Hemisphere (January)

• Isotherms often show a poleward deviation over the ocean and an equatorward deviation over continental interiors; warm ocean currents such as the Gulf Stream and North Atlantic Drift produce poleward bulges in isotherms.
• Large equatorward bends of isotherms over continents indicate strong continental cooling and penetration of polar air masses (e.g., Siberia).
• Extremely low temperatures are recorded over northern Siberia and Greenland in January.

Southern Hemisphere (January)

• The southern hemisphere shows more regular isotherms and a dominant oceanic influence; the belt of highest temperatures in austral summer commonly lies near 30°S in many oceanic longitudes.
• During southern summer the thermal equator shifts southward with the Sun's vertical ray.

Seasonal distribution - July

• July is summer in the Northern Hemisphere and winter in the Southern Hemisphere; isothermal patterns are the reverse of those seen in January.
• In July the isotherms are generally closer to parallels of latitude in many oceanic regions; equatorial oceans commonly record sea-surface temperatures exceeding 27°C.
• Continental interiors, especially in subtropical Asia, may show land surface temperatures exceeding 30°C along about 30°N latitude.

Northern Hemisphere (July)

• Continental interiors can exhibit very large annual and diurnal ranges; parts of northeastern Eurasia show ranges in excess of 60°C between extreme winter and summer values because of strong continentality.
• Poleward bulges of isotherms over continents in summer indicate intense heating of landmasses and the penetration of warm tropical air masses into higher latitudes.
• Over northern oceans, equatorward shifts of isotherms show the moderating influence of cooler seas on adjacent regions; Greenland remains among the coldest locations.
• High temperature belts commonly run through northern Africa, West Asia, north-west India and parts of the southeastern United States.

Southern Hemisphere (July)

• The Southern Hemisphere shows more regular isotherm patterns in winter with a slight equatorward bend along continental margins; the thermal equator tends to lie north of the geographic Equator in many longitudes during this period.

Vertical distribution of temperature

• In the troposphere the general tendency is for temperature to decrease with height; this decrease is referred to as the lapse rate.
• The normal (environmental) lapse rate is typically taken as about 6.5°C per kilometre, but actual lapse rates vary with humidity and local conditions.
• At the tropopause the lapse rate approaches zero and temperature becomes approximately constant with height; above the tropopause in the lower stratosphere the temperature may increase with altitude due to absorption of solar ultraviolet radiation by ozone.

Temperature anomaly

• Temperature anomaly (or thermal anomaly) is the difference between the mean temperature of a place and the mean temperature expected for its parallel (latitude).
• Largest temperature anomalies are typically observed in the Northern Hemisphere because of its greater land area and stronger continental effects; the Southern Hemisphere shows generally smaller anomalies owing to its larger oceanic area.

Mean thermal equator

The thermal equator is an isotherm that joins points of highest average annual temperature for each longitude. The thermal equator does not coincide exactly with the geographical Equator because of land-sea distribution, mountain barriers and ocean currents which create regional departures from a simple latitudinal temperature pattern.

The thermal equator shifts north and south seasonally following the Sun's vertical rays. Over the annual average it often lies slightly north of the geographic Equator; the long-term mean position of the thermal equator is commonly near about 5°N latitude. This northward bias is because the seasonal northward displacement of maximum heating during Northern Hemisphere summer is typically greater than the southward displacement during Southern Hemisphere summer, partly because of the larger land area in the north.